Oxidation-resistant ferritic stainless steel

ABSTRACT

A ferritic stainless steel having the composition: CARBON0.03% MAX. MANGANESE 0.50% MAX. SILICON0.10% MAX. PHOSPHORUS0.025% MAX. SULFUR0.025% MAX. CHROMIUM2.75 TO 6.25% ALUMINUM 5.0 TO 7.0% MOLYBDENUM2% MAX. IRONBALANCE PLUS INCIDENTAL IMPURITIES, WHEREIN THE PERCENT CHROMIUM PLUS 2 TIMES THE PERCENT MOLYBDENUM IS AT LEAST EQUAL TO THE INTEGER 5. The steel has good hightemperature oxidation resistance at all temperatures up to about 2,200*F and has sufficient ductility to be hot and cold worked into sheet and strip.

United States Patent [191 Brickner 1 July 8,1975

[ OXIDATION-RESISTANT FERRITIC STAINLESS STEEL [75] Inventor: Kenneth G.Brickner, Pittsburgh,

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

[22] Filed: Oct. 30, 1970 [21] App]. No.: 85,738

FOREIGN PATENTS OR APPLICATIONS 476,115 12/1937 United Kingdom 75/124Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J.Steiner Attorney, Agent, or FirmForest C. Sexton [57] ABSTRACT Aferritic stainless steel having the composition:

carbon 0.03% max. manganese 050% max. silicon 0.10% max. phosphorus0.025% max. sulfur 0.025% max. chromium 2.75 to 6.25% aluminum 5.0 to7.0% molybdenum 2% max. iron balance plus incidental impurities,

wherein the percent chromium plus 2 times the percent molybdenum is atleast equal to the integer 5. The steel has good high-temperatureoxidation resistance at all temperatures up to about 2,200F and hassufficient ductility to be hot and cold worked into sheet and strip.

1 Claim, No Drawings 1 OXIDATION-RESISTANT FERRITIC STAINLESS STEELBACKGROUND OF THE INVENTION This is a need in industry for bettereconomical hightemperature oxidation-resistant steels for use inapplications where high-strength at elevated temperatures is notrequired. In these applications, such as furnace parts, heat exchangerparts, automobile anti-smog devices and the like, ferritic stainlesssteels, particularly AlSl Types 430 and 446, are frequently used.Although these steels are lower in cost than comparable chromium-nickelaustenitic stainless steels, their cost is still relatively high becauseof the high chromium content and their service temperatures are limited.Specifically, Type 430 steel contains about 17 percent chromium and hasa maximum service temperature of about l,500F. Type 446 steel containsabout 25 percent chromium and has a maximum service temperature of about2,000F.

In addition to the ferritic stainless steels, high aluminum steels havebeen developed for comparable hightemperature applications. These highaluminum steels (i.e., above 4 percent aluminum) have excellentoxidation resistance up to about 2,200F. These aluminum steels do,however, have two serious disadvantages in that they are very difficultto produce in wrought form, and they exhibit relatively high and erraticoxidation rates between about l,0OO and l,400F. These high aluminumsteels, therefore, cannot be used in applications where servicetemperatures will be in the l,lOl,400F range or where cyclictemperatures through this range may be encountered.

More recently, a combination chromium-aluminum steel has been developedwhich does not experience the unusual oxidation characteristics withinthe troublesome l,lOOl,400F range. (See for example U.S. Pat. No.3,068,094, Alloy of Iron, Aluminum and Chromium, Zackay et al.) In spiteof its good oxidation resistance, this alloy is not commerciallyattractive because to impart any useful degree of ductility, the alloymust be rigidly deoxidized and degassed by costly and time-consumingvacuum melting procedures or complex chemical procedures.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a new chromium-aluminum alloy which is not onlyoxidation-resistant at all temperatures up to about 2,200F but whichpossesses a high degree of ductility without complex deoxidation ordegassing. The ductility of this inventive alloy is sufficient to permitcold rolling of the alloy to sheet and strip products in accordance withconventional cold rolling practices. In addition, this inventive steelutilizes lesser amounts of alloy additives, notably chromium, and istherefore more economical than the comparable prior art alloy discussedabove.

It is therefore an object of this invention to provide a new, low-cost,high-temperature oxidation-resistance stainless steel for use inapplications where high elevated temperature strength is not required.

Another object of this invention is to provide a new chromium-aluminumsteel alloy having good oxidation resistance at all temperatures up toabout 2,200F, and further having sufficient ductility when produced byconventional steelmaking practices to be cold rolled into sheet andstrip products.

A further object of this invention is to provide a new, high-temperatureoxidation-resistant, chromiumaluminum stainless steel alloy whichutilizes less chromium and is therefore more economical than similarprior art alloys.

These and other objects and advantages will become apparent from thefollowing detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The prior art chromium-aluminumhigh-temperature steel alloy, mentioned above, contains 7.0 to 8.0percent chromium and 6.5 to 8.0 percent aluminum. The balance thereof isof course iron with incidental impurities. Because of the high alloycontent, this steel cannot be cold rolled without excessive edgecracking when produced in accordance with conventional steelmakingpractices. In order to have any useful degree of ductility, this alloymust have exceptional low oxygen and gas contents, i.e., such low gaslevels as can only be achieved by vacuum melting practices or byspecialized chemical degassing procedures.

As noted above, the alloy of this invention is a similarchromium-aluminum steel alloy, but further having exceptional ductilityeven when produced in accordance with conventional steelmakingpractices. The crux of this invention resides in my discovery thatslight reductions in the alloy content to levels below certain criticallimits will yield such a ductile alloy without any marked sacrifice inthe desired high-temperature oxidation resistance. Stated simply, theessential critical feature of this inventive alloy is the restriction ofchromium contents to not more than 6.25 percent, and the restriction ofaluminum contents to not more than 7.0 percent. In its broadest aspect,the composition limits of this alloy are as follows:

carbon 0.03% max. manganese 0.50% max. silicon 0.10% max. phosphorus0.025% max. sulfur 0.025% max. chromium 2.75 to 6.25% aluminum 5.0 to7.0% molybdenum 2% max.

iron balance with incidental impurities.

In addition to the above ranges, the alloy of this invention is furtherlimited to the relationship in which the percent chromium plus two timesthe percent molybedenum is at least equal to the integer 5. In theabsence of molybdenum, therefore, the inventive alloy must contain from5 to 6.25 percent chromium. With given additions of molybdenum up to 2percent, however, chromium reductions equal to twice the molybdenumcontent can be tolerated, down to a minimum of about 2.75 percentchromium.

Considering the embodiment of this invention in detail, the more basicembodiment contains only chromium and aluminum without molybdenum, ashas been the prior art practice. Unlike the prior art practice, however,this embodiment of the invention contains not more than a critical 6.25percent chromium, for a chromium content of from 5.0 to 6.25 percent. Ina like manner, the aluminum content of this alloy must be restricted tothe range 5.0 to 7.0 percent. As has been noted, this contrasts withprior art limits of 7.0 to 8.0 percent chromium and 6.5 to 8.0 percentaluminum. 1n limiting the alloy additives to such lower levels, I havefound that there is not a marked sacrifice in the alloyshigh-temperature oxidation resistance. Because of these possible loweralloy additions while maintaining useful high-temperature oxidationresistance, the economic advantages of this invention become readilyapparent.

Another advantage more significant than economy is that of ductility.That is to say, I have discovered that if the chromium, aluminum andnormal residual elements are all maintained below given critical maximumlimits, the alloy will have a rather high degree of ductility. In fact,the alloy will possess sufficient ductility to be readily cold rolledinto sheet and strip products using conventional commercial equipmentand procedures. Of most significance is the fact that this ductility isachieved without any highly special processing to degas or purify thealloy. To achieve this ductility the chromium content in the alloy mustnot exceed a critical 6.25 percent, the aluminum content must not exceeda critical 7.0 percent and the residual elements must be limited asfollows: carbon, 0.03 percent maximum; manganese, 0.5 percent maximum;silicon 0.10 percent maximum; phosphorus, 0.025 percent maximum; andsulfur, 0.025 percent maximum. In order to retain a useful degree ofhigh-temperature corrosion resistance, however, at least 5.0 percentchromium and 5.0 percent aluminum must be provided. In some embodimentsof this alloy, molybdenum may be present in amounts not exceeding 2percent.

Although the above discussed composition limits will provide an alloyhaving good high-temperature oxidation resistance and exceptionalductility as stated, there are certain more preferred embodiments ofthis inventive alloy which will provide even better properties forspecific given applications. Another embodiment of this invention,therefore, is an alloy substantially as described above, but furthercontaining molybdenum in amounts not exceeding about 2 percent, andpreferably about 1 percent molybdenum. When molybdenum is in the alloyin quantities exceeding about 2 percent, even with substantially reducedchromium contents, an alloy is produced that is difficult to hot workand that has a marked tendency towards edge cracking when cold rolled tosheet thicknesses. To assure ductility, therefore, molybdenum contentsin excess of about 2 percent should be avoided, and molybdenum contentsof about 1 percent or less are preferred.

In discussing the basic embodiment, it was stated that chromium contentsof at least about 5.0 percent were essential in order to achieve anyuseful degree of hightemperature oxidation resistance. In these latterembodiments, however, where molybdenum is added to the alloy, as littleas 2.75 percent chromium can be used without sacrificing the alloyshigh-temperature oxidation resistance. More specifically, l have foundthat such small quantities of molybdenum are twice as effective aschromium for imparting high-temperature oxidation resistance in thepresence of at least 2.75 percent chromium. Hence, chromium reductionsto values between 2.75 and 5.0 percent can be realized without sacrificein oxidation resistance if half that chromium reduction is replaced withmolybdenum. Broadly stated, therefore, the alloy of this invention islimited to compositions within the above discussed ranges wherein thepercent chromium plus two times the percent molybdenum is at least equalto the integer 5. For example, a molybdenum-free alloy having 6.25percent chromium is comparable, for the objectives of this invention, toan alloy having 4.25 percent chromium and 1 percent molybdenum, or to analloy having 5.25 percent chromium and 0.5 percent molybdenum and so on.It is not essential, however, that the chromium content be reducedproportionally, or at all, when molybdenum is used. Insofar as ductilityand hightemperature oxidation resistance are concerned, there is nobeneficial or detrimental effect in adding the molybdenum to the basicalloy containing 5.0 to 6.25 percent chromium. In order to optimizeeconomy, however, chromium contents of from 2.75 to 5.0 percent shouldbe used when adding molybdenum.

Concerning the molybdenum-containing alloy discussed above, it wasstated that chromium contents above about 5 percent would not bebeneficial or detrimental to the alloys high-temperature oxidationresistance or ductility. I have discovered, however, the abovemolybdenum-containing alloy will possess superior resistance to certainother corrosive environments if from 5.0 to 6.25 percent chromium isprovided in addition to about 1 percent molybdenum. Therefore, a thirdpreferred embodiment of this alloy would be one containing 5.0 to 6.25percent chromium, about 1 percent molybdenum, 5.0 to 7.0 percentaluminum and of course having residual elements controlled as describedabove. Compared to the other preferred embodiments, this alloy will havesuperior resistance to corrosion in certain corrosive environments suchas automobile engine condensate that is encountered in automobilemufflers or anti-smog devices.

The following examples are presented to more graphically illustrate theadvantages of this invention, particularly in contrast with comparableprior art alloys. In this series of tests, 12 alloys were preparedhaving compositions as shown in Table I below.

TABLE I Compositions of Steels Investigated Percent Steel Grade C Mn P S51 Cr A1 Mo 1 Type 430 0.064 0.46 0.021 0.022 0.57 16.9 X X 2 Type 4460.093 0.06 0.022 0.012 0.42 23.9 X X 3 6A1 0.014 0.10 0.005 0.018 0.050.05 5.54 0.02 4 3Cr-6A1 0.012 0.14 0.009 0.009 0.08 3.02 5.96 0.005 53Cr6Al-1 Mo 0.013 0.065 0.007 0.010 0.06 2.99 6.06 1.04 6 3Cr-6A1-3Mo0.012 0.084 0.008 0.008 0.07 3.03 6.06 2.92 7 6Cr-6Al 0.012 0.10 0.0080.012 0.09 6.04 6.62 0.006 8 6Cr-6Al-1Mo 0.013 0.062 0.004 0.013 0.076.01 6.54 0.97 9 6Cr-6Al-3Mo 0.015 (0.049 0.012 0.06 6.03 6.55 2.97

.008 10 6.5Cr-7.3Al 0.022 0.22 0.023 0.021 0.026 6.48 7.32 X 11 7Cr-7A10.018 0.35 0.023 0.022 0.05 7.10 6.97 X 12 6Cr-6A1 0.010 0.35 0.0100.010 0.07 6.03 5.70 X

x Not determined; residual amounts only.

As noted in the above table, Steel 1 was a conventional AISI Type 430stainless steel, Steel 2 was a conventional AISl Type 446 stainlesssteel, Steel 3 was a high-temperature aluminum steel as known in theprior art, and Steels and 11 were chromium-aluminum steels havingcompositions in accordance with prior art teaching. but not speciallydegassed. Steels 4 through 9 and 12 were steels having compositions inaccordance with this invention, except that Steels 6 and 9 hadmolybdenum contents in excess of that taught and Steel 4 containedinsufficient chromium without molybdenum. Hence Steels 5, 7, 8 and 12had compositions completely within the scope of this invention.

Steels l through 9 shown in TAble I above were identically tested forhigh-temperature oxidation resistance. For this test, identical sheetsamples were exposed for eight days in a furnace through which air wascirculated by natural convection. The samples were tested at each of thefollowing temperatures: 1,200, 1.500,l .800,2,000 and 2,200F. Afterexposure, the samples were carefully descaled and the amount of weightlost by each sample was determined. The weight-loss data so obtainedestablished quantitatively the oxidation resistance of each steel. Asmall weight loss by a steel indicates good oxidation resistance,whereas a large weight loss indicates poor oxidation resistance. Theresults of this oxidation test are shown in Table 11.

TABLE II Results of Laboratory High-Temperature Air Oxidation TestsWeight Loss, mg/sq in., after 8 days exposure at Notes: The results arethe average of two tests. except where noted. NT not tested.

'- average of 4 tests.

range obtained in 8 tests.

As shown in Table 11 above, Steel 1, the Type 430 steel. had smallweight losses at 1,200 and 1,500F, but

weight loss increased markedly at 1,800F. Similarly,

Steel 2, the Type 446 steel, had a small weight loss at 1,500F but, asexpected, the weight loss increased substantially at 2,000 and 2,200F.Steel 1 was not tested at 2,000 and 2.200F because the sample would havebeen completely oxidized at these temperatures. Steel 3, the 6 percentaluminum steel, exhibited relatively high and erratic weight losses at1,200F and substantially lower weight losses between l,500 and 2,200F.Steel 4, containing 3 percent chromium and 6 percent aluminum, did notexhibit weight losses appreciably different from those of Steel 3.However, the addition of 1 percent molybdenum to such a steel as Steel3, i.e., Steel 5, resulted in a substantial reduction in weight loss at1,200F. The weight losses of this steel at 1,500 and 1,800F were alsoslightly lower than those of Steel 3. At 2,000F, however, Steel 5exhibited a weight loss somewhat higher than Steel 3 but markedly lessthan Steel 2 at this temperature. In Table II, it can be seen that Steel7, with 6 percent each of chromium and aluminum, was the best sample foroxidation resistance over the total range of temperatures.

Table II further shows that the addition of 1 percent molybdenum to a 6percent chromium-6percent aluminum alloy (Steel 8) had no appreciabledetrimental or beneficial effect on oxidation resistance. however, 3percent molybdenum in such a steel (Steel 9) resulted in a relativelyhigh weight loss at 1,800F. In addition, Steel 9, as well as Steel 6,was difficult to hot work, and cold rolled sheets thereof had markedtendencies toward edge cracking.

As already noted, Steels 6 and 9, having 3 percent molybdenum, werequite difficult to hot roll, and had marked tendencies towards edgecracking when cold rolled. In addition, Steels 10 and l l, which areoutside the scope of this invention, exhibited deep transverse cracks in1 inch plates that were hot rolled at 2,200F from slab ingots. Incontrast, Steel 12, which is within the scope of this invention, was hotrolled in the same manner without difficulty. Portions of the 1 inchplates of Steels 10 and 11 that did not contain cracks were subsequentlyhot rolled at 2,200F to 0.225-inch plates, and all the resulting platesexhibited a moderately slivered surface. Steel 12, which was hot rolledidentically, was essentially free of slivers. The 0.225 inch hot rolledplates were then annealed at 1,500F and descaled by grit blasting andthen cold rolled. Steel 10 cracked on cold rolling to a thickness of0.204 inch, and Steel 11 cracked on cold rolling to a thickness of 0.098inch. In contrast, Steel 12 was cold rolled to 0.096 inch completelywithout difficulty. Similarly, Steels 5, 7 and 8, which are also withinthe scope of this invention, were hot rolled and double cold rolled withan intermediate anneal to 0.065 inch thick completely withoutdifficulty.

I claim: 1. A ferritic stainless steel consisting essentially of carbon0.03% max. manganese 0.50% max. silicon 0.10% max. phosphorus 0.025%max. sulfur 0.025% max. chromium 2.75 to 5.0% aluminum 5.0 to 7.0%molybdenum about 1% iron balance plus incidental impurities.

wherein the percent chromium plus two times the percent molybdenum is atleast equal to the integer 5, said steel characterized byhigh-temperature oxidation resistance at all temperatures up to about2,200F, and sufficient ductility to be hot and cold worked into sheetand strip products.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5, 95,49 Dated y 1975 O Inventofls) Kenneth G. Briokner' It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In Table 1, Steel 9, P should be 0.008

S should be 0.012

' Si should be 0.06 Cr should be 6.03

A1 should be 6.55

Mo should be 2.97 O

Between lines 9 and 10 of Table 1, under Mn, delete Signed and Scaledthis seventh Day of 0mm 1975 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN' Arresting Officer Commissionernj'Parents and Trademarks FORM PO-1050 (10-69) USCOMM DC 603764369 U.S.GOVERNMENT PRINTING OFFICE: 8 69. 93 o

1. A FERRITIC STAINLESS STEEL CONSISTING ESSENTIALLY OF